RF-excited, hermetically sealed, pulsed CO2 lasers are gas discharge lasers widely used in material processing and laser machining applications such as via hole drilling in printed circuit boards and glass-plate scribing for TV screen manufacture. Such a laser includes a laser gas mixture including CO2 and inert gases. A gas discharge is ignited in the laser gas to energize the CO2 for providing optical gain. In order to be adaptable to a variety of applications, such a laser should be capable of operating in a wide variety of pulse formats including a wide range of constant pulse repetition frequencies (PRF) to random sequences of changing PRF. An RF-excited, hermetically sealed, pulsed CO2 laser typically requires pre-ionization of the laser gas in order to provide near-immediate ignition of the discharge in response to a user command signal. Delays in this response are commonly referred to as “pulse-time jitter” by practitioners of the art.
In an RF-discharge gas laser the RF resonant circuit (which includes the lasing gas between discharge electrodes) has a high Q and a correspondingly high resonant frequency when the discharge is un-lit. High Q is associated with high impedance at resonance. Once the discharge is lit, the Q drops significantly and the resonant frequency of the RF circuit drops correspondingly. It is easier to achieve ignition of a gas discharge with a high-Q resonant circuit than with a low-Q resonant circuit. This resonant frequency-shift presents a problem in the design of RF excited CO2 lasers, as the frequency of the RF supply to the electrodes must be selected to provide a compromise between optimum ignition effectiveness and efficiency of operation once the discharge is ignited (lit). The problem is complicated by the fact the longer a discharge is not lit the more difficult it is to reignite the discharge. Various approaches have been taken by CO2 laser manufacturers to alleviate this problem.
One recent approach to alleviating the problem involves using a frequency agile RF power supply with an ability to detect the RF resonant frequency with the discharge in un-lit and lit conditions. This is described in U.S. Provisional Application No. 61/057,392, filed May 30, 2008, assigned to the assignee of the present invention. Dual frequency approaches are described in U.S. Pat. No. 6,181,719, and in U.S. Pat. No. 5,150,372.
A limitation of the approaches referenced above is that as the area of the discharge increases, i.e., the area of the discharge electrodes increases, the difference between the resonant frequencies of the lit and un-lit discharge conditions increases. For high-power, slab-type CO2 lasers, for example CO2 lasers having an output power of about 1 kilowatt (kW) or greater, this resonant-frequency difference becomes a significant fraction of the center (nominal) frequency of an RF power supply. By way of example, for a center frequency of 100 MHz, representative of the optimum lit resonant frequency, the unlit discharge resonant frequency can be lower by approximately 25 MHz. This places a difficult design burden in providing an adequate bandwidth to accommodate the frequency difference.
Turning now to a discussion of prior-art pre-ionization approaches, an early prior art approach, described in U.S. Pat. No. 5,434,881, involved the use of an igniter, including an electrode similar to that of a spark plug. This approach requires a separate power supply and a hermetically sealed penetration into the gas enclosure of the laser. These add cost and complexity to a laser. Further, the approach presents a problem inasmuch as the lifetime of the laser is compromised by ablation of the electrode material into the laser gas mixture.
Improvements directed to minimizing the electrode ablation problem are described in U.S. Pat. No. 6,963,596, and in U.S. Pre-Grant Publication No. 2008/0069170, each thereof assigned to the assignee of the present invention. These improvements, however, still require a separate power supply and a hermetically sealed penetration into the gas enclosure of the laser. The above cited patents, published applications and provisional applications are hereby incorporated herein by reference.
One prior-art pre-ionization approach that does not require a separate igniter or a separate power supply involves using the discharge electrodes of the laser to provide pre-ionization in addition to powering the lasing discharge. In this approach, the RF power supply is operated at a single frequency corresponding to the lit-discharge resonant frequency. Pre-ionization is provided by operating the power supply with RF pulses delivered at a pre-selected PRF and short enough in duration not to cause laser action, but long enough in duration to generate sufficient free electrons in the lasing gas to provide the pre-ionization. The pulse length is increased in response to a command signal to provide the main lasing discharge. The pre-ionization discharge is referred to by practitioners of the art as a “simmer-discharge’.
This simmer-discharge approach can be effected by appropriately programming a gate array chip in the RF power supply that is usually contained within a power supply to perform status monitoring tasks or for turning off the power supply if a large voltage standing wave ratio (VSWR) is detected. A large VSWR occurs if the discharge does not light on a user command. A large VSWR can damage the power supply if the power supply is not turned off.
FIG. 1 schematically illustrates a basic logic arrangement 10 of the above-discussed prior-art pulsed simmer-discharge approach. The logic arrangement includes a logic OR-gate 12, and RF-power supply 18. Short duration simmer command pulses are delivered to input 14 of the OR-gate and from the OR-gate to the RF power supply. The simmer command pulses cause the RF-power supply to deliver simmer RF-pulses of corresponding duration and PRF to laser discharge electrodes (not explicitly shown).
The duration and PRF of the simmer command pulses and corresponding RF simmer pulses are experimentally determined and selected for a given laser design such that the corresponding RF-pulses create and maintain free electrons in the lasing gas to provide pre-ionization but do not excite the lasing gas sufficiently to induce laser action. By way of example, the simmer pulses may have a duration of about 4 microseconds at a PRF of about 1 kilohertz (kHz). The duration and PRF of the simmer pulses are also experimentally selected to minimize time-jitter in lighting the discharge whenever a user command pulse occurs in between the simmer pulses.
Laser-discharge command pulses (user command pulses) are delivered to input 16 of the OR-gate and from the OR-gate to the RF-power supply causing the RF-power supply to deliver corresponding laser-discharge RF-pulse to the laser electrodes. The laser-discharge command pulses, and laser-discharge RF pulses have a much longer duration than the simmer pulses. Typically the simmer and laser-discharge RF pulses will have the same peak power. The PRF of the user command pulses can vary randomly while the PRF of the simmer pulses is fixed, for example at about 1 kHz as discussed above.
One problem with the arrangement of FIG. 1 is that the delivery-frequency to OR-gate 12 of simmer pulses and user command pulses is typically asynchronous. This asynchronous relationship between the simmer and user command pulses causes an amplitude modulation or a pulse width modulation (dependent on the type of RF-power supply employed) to occur on the laser output pulses due to the varying timing relationship between the simmer-discharge command pulses and the laser-discharge command pulses in the composite signal issued by the OR-gate 12. A description of this relationship is set forth below with reference to FIGS. 2A-C.
FIG. 2A is a graph schematically depicting voltage as a function of time for a simmer-discharge command pulse train in the arrangement of FIG. 1. Pulses have a duration WS and sequential ones thereof are temporally separated by a time period TS. The PRF of the pulses is 1/TS. FIG. 2B is a graph schematically depicting voltage as a function of time for a user (lasing) command pulse train in the arrangement of FIG. 1. Pulses have a duration WU, and sequential ones thereof are temporally separated by a time period TU. The PRF of the pulses is 1/TU, which in this example is greater than 1/TS. FIG. 2C is a graph schematically illustrating laser output pulses as a function of time that would result from the pulse trains of FIGS. 2A and 2B being delivered to a linear RF power supply causing the RF power supply to deliver corresponding RF pulses to the discharge electrodes. At times T1, T4, and T7, when only simmer-discharge pulses are delivered to the electrodes there is no corresponding laser output. At times T2, T3, T5, T6, and T8, the power of the simmer pulses is combined with the power of laser-discharge pulses. As the laser-discharge pulses cause laser output, the laser output power is increased during the period of delivery of the simmer pulse in combination with the laser-discharge pulse, as schematically depicted, causing unwanted amplitude modulation of the output pulses. Gas kinetics of the discharge will modify somewhat the schematic laser output waveform shown in FIG. 2C. The smaller TS is with respect to TU, the higher the frequency of the modulation. The smaller Ws is with respect to Wu, the less pronounced will be the modulation and visa versa.
It should be noted that FIG. 2C depicts modulation that occurs when a liner RF-power supply is used in the circuitry of FIG. 1. A linear power supply has an output amplitude that varies dependent on the amplitude of an input command signal. A more commonly used RF-power supply is referred as a digital RF power supply the output amplitude of which is fixed and independent of the amplitude of the command signal. Were such a power supply used in the circuitry of FIG. 1, pulse width modulation would occur whenever simmer pulses occurred near the beginning or end of a user command pulse. In this case the simmer pulses add to the duration of the discharge pulses thereby increasing the amount of energy in discharge pulses and correspondingly energy deposited on a workpiece.
Pulse-width modulation is as undesirable as amplitude modulation in laser processing. There is a need for a simmer-discharge arrangement for providing pre-ionization that does not lead to any form of modulation of the laser output pulses.